Optical data storage disks have gained widespread acceptance for the storage, distribution and retrieval of large volumes of information. Optical data storage disks include, for example, audio CD (compact disc), CD-R (CD-recordable), CD-RW (CD-rewritable), CD-ROM (CD-read only memory), DVD (digital versatile disk or digital video disk), DVD-RAM (DVD-random access memory), and various other types of writable or rewriteable media, such as magneto-optical (MO) disks, phase change optical disks, and others. Some newer formats for optical data storage disks are progressing toward smaller disk sizes and increased data storage density. Many new formats boast improved track pitches and increased storage density using blue-wavelength lasers for data readout and/or data recording. A wide variety of optical data storage disk standards have been developed and other standards will continue to emerge.
Optical data storage disks are typically produced by first making a data storage disk master that has a surface pattern that represents encoded data on the master surface. The surface pattern, for instance, may be a collection of grooves or other features that define master pits and master lands, e.g., typically arranged in either a spiral or concentric manner. The master is typically not suitable as a mass replication surface with the master features defined within an etched photoresist layer formed over a master substrate.
After creating a suitable master, the master can be used to make a stamper, which is less fragile than the master. The stamper is typically formed of electroplated metal or a hard plastic material, and has a surface pattern that is the inverse of the surface pattern encoded on the master. An injection mold can use the stamper to fabricate large quantities of replica disks. Also, photopolymer processes can be used with stampers to fabricate replica disks. In any case, each replica disk may contain the data and tracking information that was originally encoded on the master surface. The replica disks can be coated with a reflective layer, a dye layer, and/or a phase change layer, and are often sealed with an additional protective layer. Other media formats, such as magnetic disk formats, may also use similar mastering and stamping techniques, e.g., to create media having small surface features which correspond to magnetic domains.
In some cases, the surface pattern encoded on the data storage disk master represents an inverse of the desired replica disk pattern. In those cases, the master is typically used to create a first-generation stamper, which is in turn used to create a second-generation stamper. The second-generation stamper, then, can be used to create replica disks that contain an inverse of the surface pattern encoded on the master. Creating multiple generations of stampers can also allow for improved replica disk productivity from a single data storage disk master.
The mastering process is one of the most critical stages of the data storage disk manufacturing process. In particular, the mastering process defines the surface pattern to be created in replica disks. The master will pass on any variations or irregularities to stampers and replica disks, and therefore, the creation of a high quality master is important to the creation of high quality replica disks. For this reason, it is highly desirable to improve mastering techniques.
The mastering process commonly uses a photolithographic process to define the master surface pattern. To facilitate the mastering process, an optically flat master substrate is coated with a layer of photoresist. A tightly focused laser beam passes over the photoresist-coated substrate to expose grooves or other latent features in the photoresist, which may be categorized as a direct-write photolithographic technique. The focused beam may also be modulated or wobbled to define information such as encoded data, tracking servos, or the like, within the features of the master disk. After exposing the photoresist, a developer solution removes either the exposed or unexposed photoresist, depending on whether a positive or negative photoresist material is used. In this development step, the latent exposure pattern is manifest as a topographical master pattern.
One source of mastering noise may be due to stray light caused by reflections of the incident laser light from the photoresist-coated substrate. The stray light may expose unwanted regions of the photoresist causing decreased sharpness and resolution of the mastered features during the development step. The master will then pass these noisy features on to stampers and replica disks.